In many ways it's the opposite, the code counts as writing. At least where I live (should be similar in other EU countries) from the perspective of copyright law code is largely classified as literary work. There are few special rules related to fact that code is typically written by large team employed by company compared to books being written by single author and printed by publisher. Also few special rules that don't make sense for regular literary works related to reverse engineering, interface compatibility with other software and copying as part of installation/execution process.
All the code specific licenses are arbitrary rules decided by right owners under which they license others to use their work. Book authors typically don't have to worry whether when granting permission to read their work they also need to grant rights to relevant patents, or whether dynamic or static linking should be permitted.
Assuming Microsoft owns all the necessary rights for everything I don't see a major reason they couldn't release it as single work under whatever license they want. Considering the way this specific game is written I don't think you can even cleanly separate code from rest of the writing. It's all one big program with bunch of short string literals sprinkled all over the place. You could take all the string literals, but without code defining how those strings are ordered in a non linear and interactive way it would make as much sense as reading a book where all the sentences have been sorted in alphabetical order.
I doubt any of the Zork authors were part of writer union and negotiated separate licensing rules for the text they wrote.
With regards to common case where source is released with open source license separate from art assets there are a couple of reasons. It's much easier to separate pictures and music from code and tell that those are covered by different license. For some of the art assets especially music it's not uncommon that the publisher themselves don't have full rights to them and only have limited license to use in game but not to relicense as separate works. Releasing source code without art assets makes it easier to maintain commercial value and limit ability for others to exploit the work as ready to use product (or sequals) while still allowing programmers to study the code and learn from the technical tricks in code or making new games based on same engine. If I am not mistaken at
From the historical preservation perspective art assets are less likely to bitrot and become unusable. 20 years in future you will likely still be able to rip them out of commercial builds of game with little losses and worst case observable as standalone media. But the code is a lot more likely break and not be runnable on future OS/hardware. Code also has a lot more hidden aspects that you can much more easily observe by reading source code directly instead of reverse engineering the compiled executables. Better release the source code while it hasn't been completely lost.